Being fair to both children doesn't always mean giving the same thing to both. For example, Alex and I wouldn't be equally happy with a nice My Little Pony or a stupid set of Magic cards -- if we both got one, only one of us would be happy.
I tried to bring this up in class a couple months ago, when we were discussing designing experiments to test linguistic judgments. Say you want to compare how acceptable people find (1) vs. (2):
(1) She is easy to fight.
(2) She is eager to fight.
But you want to precede these sentences with a context where "she" is introduced, like "Mary is coming over, and she is really mad." That sortof makes sense with both, but it makes more sense with (2) than with (1). In order to avoid introducing variables, you might be tempted to use the same context for both sentences, but there's a trade-off. Maybe to really be "fair", you should use different contexts for both sentences. Then you could ensure that the transition between the context and the target sentence is always equally coherent.
Speaking of coherence, is this making sense?? No one in class understood what I was talking about.